1. Field of the Invention
The present invention relates to a process for producing a columbite-type niobate and a process for producing a niobium-containing lead-type perovskite compound therefrom. More particularly, the invention relates to a process for producing which permits transition into columbite-type crystals and transition into perovskite-type crystals at relatively low temperatures.
2. Prior Art
Ceramic materials of the perovskite-type have been widely used as piezo-electric materials, pyroelectric materials and ferroelectric materials, and there have been used various perovskites having chemical compositions represented by a general formula (2), EQU ABO.sub.3 ( 2 )
A representative example of the perovskite may be a niobium-containing lead-type perovskite such as a lead magnesium niobate (PMN) which is a composite perovskite in which the site A in the above formula (where M denotes a divalent or trivalent metal).
A so-called solid-phase process has been known for synthesizing a composite perovskite of this type, according to which MgO and Nb.sub.2 O.sub.5 are mixed together using a ball mill and are reacted together at 1000.degree. C. in solid phase to form an MgNb.sub.2 O.sub.6 (columbite) which is then mixed together with PbO in a ball mill followed by calcining at 850.degree. to 900.degree. C. to prepare a perovskite-type lead magnesium niobate.
According to the above-mentioned process, however, the columbite that is formed and PbO react with each other in the solid phase at considerably high temperatures only at which, however, the PbO volatilizes, resulting in nonhomogeneous composition or making it difficult to accomplish the composition as desired.
In order to solve the above problem, Japanese Patent Publication No. 34890/1990 discloses a process for the preparation a composite perovskite crystal powder comprising adding an organic solvent and an alkali solution to the niobium pentachloride or the niobium alkoxide to form a niobium hydrogel, and adding one or more kinds of metal salt aqueous solutions thereto, followed by drying and calcination.
Furthermore, Japanese Laid-Open Patent Publication No. 83628/1986 discloses the preparation of a PMN perovskite by spraying into an electric furnace maintained at 1000.degree. C. a solution that is obtained by mixing an acidic solution of oxalic acid of niobium and metal magnesium and the like together, calcining the resulting powder at 900.degree. C. for three hours to obtain an MgNb.sub.2 O.sub.6 single phase, mixing the MgNb.sub.2 O.sub.6 single phase with PbO together in a pot mill, and calcining the mixture at 850.degree. C. for one hour.
According to the former proposal, the PMN can be formed at a temperature which is considerably lower than that of the solid-phase process but still requiring a temperature of about 800.degree. C. and making it necessary to handle the whole components inclusive of lead component in the form of solutions.
The latter proposal has an excellent idea in that the columbite is prepared by the wet-type process and the PMN is synthesized therefrom by the dry-type process making, however, it necessary to effect the processings at high temperatures such as decomposition at 1000.degree. C. for forming the columbite, calcining at 900.degree. C., and calcining into PMN at 850.degree. C., thus leaving much room for improvement.